Add validation function internals to a trace in case it's failed

[avpotapov00]

Sometimes, a validation function may compute something based on the data structure content (for example, extracting two ints from one long). In that case, it would be convenient to have the ability to stop somewhere in the validation function to see the validation function state in the debugger. But if a test fails for any reason except validation function failure, printing the validation function makes no sense, just making the output more verbose. Let's show the validation function trace in the Lincheck console output and the plugin, but only when it's failed.